Qualitative bacterial culture of specimens taken from several operative sites during abdominal hysterectomy has not been of value in predicting postoperative infection. We have therefore studied the relationship between the magnitude of contamination and the likelihood of the development of postoperative infection, in the course of a trial comparing the efficacy of cefotetan with ampicillin-plus-metronidazole for chemoprophylaxis in 163 women undergoing abdominal hysterectomy. Forty women who did not receive chemoprophylaxis were also studied. From each patient, an intraoperative sample of pelvic fluid was obtained after closure of the vaginal vault and examined quantitatively for anaerobic, aerobic and microaerophilic bacteria. The incidence of postoperative infectious morbidity (wound infection and febrile morbidity) was correlated with bacterial counts. Analyses by step-up multiple logistic regressions were performed on all the variables and only the total and microaerophilic bacterial counts were significant. Of the 40 patients with total bacterial counts greater than or equal to 10(4) cfu ml-1, 42.5% developed postoperative infectious morbidity, compared with 12.6% of 135 of patients with counts of less than or equal to 10(3) cfu ml-1. The contaminating bacteria were similar to those of the vaginal and skin flora, with anaerobes occurring in 52.9% and microaerophiles in 63.9% of positive fluid samples. Thus, we have concluded that the magnitude of contamination during abdominal hysterectomy is an important determinant in the development of postoperative infectious morbidity.